12 INTEODUCTION. 



this is present) by being, in these cases, always forked, the 

 absence of the discal cell being only caused through the absence 

 of a short cross-vein between the two main branches of the 4th 

 vein, and this cross-vein is usually called the discal cross-vein, on 

 account of its dominating the discal cell. It vAl\ be seen that the 

 4th longitudinal vein forks at the discal cell,* its upper or anterior 

 branch forming the basal and anterior sides of this cell; the lower 

 or posterior branch forming the posterior and outer sides of the cell. 

 Apparently, the outer or distal side of the discal cell is formed 

 in most genera of Tipuliu^ by two short veins, both nearly 

 straight themselves, yet not often in a straight line with one 

 another. The upper one of these short veins is my discal cross- 

 vein, the lower one is considered part of the lower branch of the 

 4lli vein, the straight continuation of the basal part of the 4th 

 vein being regarded as a branch vein known as Loew's posterior 

 intercalary vein. The upper branch of the 4th vein, after quitting 

 the discal cell, forks again, the upper prong being considered as 

 the continuation of the vein itself, the lower prong the branch 

 vein, and this latter is known as Loew's anterior intercalary vein."^ 

 When the anterior or upper branch of the 4th vein is forked, the 

 section between the discal cell and the fork is known as the 

 ■petiole of the 2nd posterior cell. ■ In the same manner, in a case 

 where there is no discal cell and the posterior branch of the 4th 

 vein is forked, it would be correct to speak of the corresponding 

 section of that branch as the " petiole " of that posterior cell 

 which was contained by the fork. 



Somewhere beyond the middle of the 4th longitudinal vein is a 

 longer cross-vein, generally more or less obliquely placed. This is 

 the posterior cross-vein, which in Tiptjlid^ invariably connects 

 the 4th and 5th veins, and is practically always straight itself, 



* Slightly but distinctly before it in Megistocera, the only exception known 

 to me. 



t It seems to me that although the lower prong of the upper branch of the 

 4th vein is really the additional veinlet due to the forking, a mistake has been 

 made by even Loew, one of the soundest of dipterologists, in regarding the 

 upper prong of the lower branch of the 4th vein as the true continuation of 

 that branch, since, when this lower branch is not forked it never takes the 

 course containing two sharp angles but continues in a more or less straight 

 line to the wing-margin. This compels me to believe that the luwer j>>'ony is 

 the true continuation of the loxmr hranvh o{ the 4th vein, and that the upper 

 prong is the additional veinlet due to the forking and that tliis upper proug 

 should have been named the posterior intercalary vein by Loew. I have not 

 seen this view suggested anywhere, but unless it be admitted, the anomaly 

 would be presented of the lower branch, when forked, taking two abrupt angles, 

 but when simple, proceeding in a straight line to the wing-margin, a curious 

 and quite illogical theory. This view applies to the Limnobiin.e only, as in 

 the TiPULiN.E, in which the lower branch is always forked, the lower prong 

 appeals irresistibly to me as the additional veinlet, and the upper one (forming 

 part of the discal cell and being much less angulated) as the true continuation 

 of the lower branch. There seems nothing illogical in the assumption that an 

 additional veinlet may occur on either side of a parent vein as a subfamily 

 character. 



